Bernardo Tavares Freitas

Origin and impact of the oldest metazoan bioclastic sediments

The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth’s history. The impact of bioclast origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index fossil Cloudina from the Itapucumi Group, Paraguay. Skeletal concentrations include both dense accumulations of parautochthonous, disarticulated specimens (“Type 1 deposits”) and in situ specimens preserved as loosely packed assemblages (“Type 2 deposits”). At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. The simple fabric and geometry of these accumulations are typical of Cambrian-style shell beds. Despite their Precambrian age, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation in early shell beds was determined more by the acquisition of hard parts than by environmental changes.

The ancestors of meandering rivers

Modern alluvial plains, in contrast to their pre-Silurian counterparts, are characterized by the presence of meandering rivers in low-downstream-gradient areas, constituting efficient transport systems that maintain high bottom shear stresses in deep channels, which are made possible by bank stabilization, most commonly provided by vegetation in Earth’s recent history. Here we show, through numerical modeling and field-based description of large-scale exposures in Mesoproterozoic successions, that prevegetation rivers in low-downstream-gradient areas were markedly different from both younger meandering rivers and the common prevegetation sheet-braided rivers, showing deeper braided channels and greater floodplain preservation than the latter. These systems were less frequent and had lower transport efficiency than modern meandering rivers, implying differences in global-scale Earth-surface dynamics, from the weathering of silicate minerals in floodplains to the grain size distribution in all clastic depositional systems.

Underneath the Pantanal Wetland: A Deep-Time History of Gondwana Assembly, Climate Change, and the Dawn of Metazoan Life

Underneath the wetlands of the Brazilian Pantanal are hidden key ecological and geological events of the history of our planet. In this chapter we show that Precambrian rocks forming the hills and mountains surrounding the Pantanal floodplains record (a) the cyclic process of supercontinents assembling, (b) the origin of complex life forms on Earth, and (c) the past global climate changes. It further unveiled the most recent geochronological data and paleontological and tectonic discoveries for modeling the evolution of the Pantanal basement rocks. Various questions are also addressed, including the formation time of the Rodinia and Gondwana supercontinents, the triggering factor leading to animal skeleton biomineralization, and the “Snowball Earth Hypothesis.”